a) Field of the Invention
The present invention relates to a real image mode variable magnification finder optical system designed separately from a photographing optical system and intended for use with photographic cameras, video cameras, etc, and more specifically to a real image mode variable magnification finder optical system which comprises, in the order from the object side, an objective having a positive refractive power and an eyepiece having a positive refractive power.
b) Description of the Prior Art
The inverse Galilean finder optical system is known well as a finder optical system comprising a photographing system and a finder system which are designed separately. However, the finder optical system of this type has defects that the visual field frame may not be observed clearly and that clear observation of the visual field itself is hindered due to the ghost and flare produced by the half mirror used for forming the visual field frame.
In contrast, the Keplerian finder optical system, which is adapted so as to allow observation through an eyepiece optical system of a real image formed by an objective optical system, corrects the defects of the inverse Galilean finder optical system almost completely and assures clear observation.
As examples of the Keplerian finder optical system which have a variable magnification function, there are known a zoom type finder optical system consisting of two lens components and another zoom type finder optical system whose objective system consists of three lens components. The former type is exemplified by the finder optical systems disclosed by Japanese Patents Preliminary Publication Nos. Sho 61-156018, Sho 64-65519 and Hei 1-257817. Further, known as the latter type is the finder optical system proposed by Japanese Patent Preliminary Publication No. Hei 1-131510.
However, the former type has a defect that it has a low variable magnification ratio though it permits shortening the total length of the finder section due to a fact that it has a relatively long distance as measured from the final surface of the objective to the intermediate image surface, i.e., a relatively long back focal length, and permits folding the optical path by arranging a first reflecting surface for image erection within the range of the back focal length.
On the other hand, the latter type can easily have a variable magnification ratio of 2 or higher. In addition, when attention is paid only to the wide-angle position of the latter type wherein the finder optical system has a short length as measured from the first lens component to the third lens component thereof and a long back focal length, the latter type seems to permit folding the optical path, like the former type, by arranging a first reflecting surface between the third lens component and the intermediate image surface. However, the latter type has a very short back focal length at the telephoto position thereof and does not actually permit interposing the first reflecting surface within the range of back focal length, thereby obliging to arrange the first reflecting surface after the intermediate image surface. Consequently, the latter type has a defect that it has a long distance as measured from the first lens component to the intermediate image surface and does not permit shortening the total length of the finder section.